We show that Rh substitution at the Co site in Zr{sub 0.5}Hf{sub 0.5}Co{sub 1-x}Rh{sub x}Sb{sub 0.99}Sn{sub 0.01} (0<=x<=1) half-Heusler alloys strongly reduces the thermal conductivity with a simultaneous, significant improvement of the power factor of the materials. Thermoelectric properties of hot-pressed pellets of several compositions with various Rh concentrations were investigated in the temperature range from 300 to 775 K. The Rh 'free' composition shows n-type conduction, while Rh substitution at the Co site drives the system to p-type semiconducting behavior. The lattice thermal conductivity of Zr{sub 0.5}Hf{sub 0.5}Co{sub 1-x}Rh{sub x}Sb{sub 0.99}Sn{sub 0.01} alloys rapidly decreased with increasing Rh concentrationmore » and lattice thermal conductivity as low as 3.7 W/m*K was obtained at 300 K for Zr{sub 0.5}Hf{sub 0.5}RhSb{sub 0.99}Sn{sub 0.01}. The drastic reduction of the lattice thermal conductivity is attributed to mass fluctuation induced by the Rh substitution at the Co site, as well as enhanced phonon scattering at grain boundaries due to the small grain size of the synthesized materials. - Graphical abstract: Significant reduction of the lattice thermal conductivity with increasing Rh concentration in the p-type Zr{sub 0.5}Hf{sub 0.5}Co{sub 1-x}Rh{sub x}Sb{sub 0.99}Sn{sub 0.01} half-Heusler materials prepared by solid state reaction at 1173 K.« less

Several compositions of the p-type half-Heusler alloys Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb 1–xSn x (0≤x≤0.4) were synthesized by mechanically alloying high purity elemental powders using hardened steel jars and balls on a high energy shaker mill. Powder X-ray diffraction (XRD) investigations of several aliquots taken after regularly spaced milling time suggested that single phase products with half-Heusler (HH) structure can be obtained after 10 h. However, XRD and transmission electron microscopy (TEM) studies of several specimens obtained from compacted polycrystalline powders of Z₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb 1–xSn x alloys using a uniaxial hot press (HP) revealed the presence of CoSb inclusions with various sizes embedded inside themore » HH matrix. Hall effect, electrical conductivity, and thermopower data collected between 300 K and 775 K on several compositions suggested that electronic transport in the synthesized Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb 1–xSn x/CoSb composites strongly depends on the average size and/or mole fraction of the embedded CoSb inclusions rather than the fraction (x) of Sn substituting for Sb. Among the samples investigated, the nanocomposite with x=0.2, which contains nanometer-scale CoSb inclusions, showed the largest power factor (800 μW/K² m at 775 K) and the lowest lattice thermal conductivity (~2.2 W/m K at 775 K) leading to a six-fold enhancement in the figure of merit when compared to the Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb₀.₉₉Sn₀.₀₁ bulk matrix. - Graphical abstract: CoSb nanoinclusions embedded into a p-type Zr₀.₅Hf₀.₅Co₀.₄Rh₀.₆Sb 1–xSn x half-Heusler matrix simultaneously boost the thermopower and carrier mobility leading to a drastic enhancement of the power factor of the resulting bulk nanostructured materials. Highlights: • The phase composition of half-Heusler (HH) alloy is very sensitive to the synthesis method. • Mechanical alloying (MA) of elements results in bulk HH matrix with CoSb inclusions. • CoSb nanoinclusions simultaneously boost the thermopower and carrier mobility with the HH matrix. • The MA process reduces lattice thermal conductivity due to high density of grain boundaries.« less